Since HMs are sometimes weak like Cut and Rock Smash,
you can teach HMs to just a couple pokemon. First you need to teach any water pokemon surf and dive. Go underwater and catch a clamperl. Then make the pokemon that you surfed on forget surf and dive (by the move deleter) and replace it with 2 TMs, or wait til it levels up and learns new moves. Then go in the really tall grass near the town where everyone lives in trees. Catch a tropius. Then teach the clamperl Surf, Dive, and Waterfall. Teach Tropius Cut, Rock smash, Strength, + Fly. But if you really need flash, just teach it to a weak pokemon and
take it with you when you go in caves.